Antenna



Feb. 8, 1949.

ANTENNA Filed March 14, 1946' IN VENT OR.

[@178 A Braden R. A. BRADEN 2,460,869 r i atented Feb. 8 194 1 ANTENNA Rene A. Braden, Princeton, Nu J., assignorto: Radio Corporation of America, a corporation of Delaware Application March 14, 194$,SerialNo.- 654,276

, V. 1.1 Claim. (01.250-33L65) 1 This invention relates to radio systems of the type in which radio signals are directively transmitted and received substantially simultaneously in the same directive pattern, and more particularly to improvements in the art of prevent-- ing or minimizing feed-through, i. e., the trans fer of energy directly between the transmitter and receiver. Feed-through is a problem which arises- I in radio distance measuring systems, particularly those of the frequency modulation type such as that described in U. S. Patent 2,011,392, and in other generally similar systems requiring substantially continuous transmission of signals to an object and reception of said signals after reflection from said object.

Such systems frequently include directive antennas having reflectors which may be flat, dishshaped, or paraboloidal, but which in any case are made of conductive material. Two antennas are used, one for transmission and one for reception. It is a relatively easy matter to position and orient the antennas so that the principal beam of each one does not impinge on the other, and yet have the beams substantially coincide. It is found that even when this is done, there is too much feed-through, because circulating currents in one reflector induce similar circulating currents in the other reflector, thus coupling the two antenna systems.

Although a certain amount of feed-through is permissible, and may be required in some systems, it is likely to be intolerably large when the antennas are close together. It is the principal object of the present invention to provide an improved method of and means for reducing feedthrough between closely spaced reflectors. Another object is to provide, in a system of the described type, efiective reduction of feed-through throughout a relatively wide frequency band rather than at a single frequency.

The invention will be described with reference to the accompanying drawing, wherein:

Figure 1 is a schematic perspective diagram of a typical embodiment of the invention,

Figure 2 is a sectional side elevation of one of the antennas used in the system of Figure l, and

Figure 3 is a sectional view of a modified structure according to the invention.

Referring to Figure 1, a radio transmitter l and a radio receiver 3 are connected respectively to antennas 5 and l. The antennas 5 and 1 are identical, each including in the present illustration a paraboloidal reflector 9 and a doublet radiator ll centered substantially at the focus thereof. Each of the reflectors 9 is surrounded at its rim by a resonant trap comprising a channel I; of conductive material closed at the rear andopenin the directionof the parabola.

As' shown more clearly in FigureZ, the; channel I3 is in the form of an annulus, including an annular cavity having a depth of approximately one-quarter wavelength and a width of approximately one-tenth wavelength at the mean frequency of operation of the'system. The trap may be made as a single piece with the reflector 9, as shown, or may be made separately as a U- shaped channel, then curved and wrappedaround the rim of the reflector.

It has been found in practice that antennas of the general type shown in Figure 1 (i. e. those in which directivity is provided principally by a reflector) tend to couple to each other as a result of currents induced in the reflector by the radiator and concentrated in the rim or edge of the reflector. The current flowing in the edge of the reflector of the transmitting antenna induces cur rent in the reflector of the receiving antenna, particularly at the edge portion nearest the transmitting antenna. This current in turn induces current in the radiator elements of the receiver antenna, providing feed-through from the transmitter to the receiver. 7

The above described phenomenon is minimized in the system of the instant invention by the operation of the traps l3. The outer wall of the annular channel acts as the outer conductor of a coaxial transmission line section, while the wall nearer the edge of the reflector acts as the inner conductor. Since this line section is approximately one-quarter wavelength long and is shortcircuit/ed at one end by the back wall of the channel, it presents an extremely high impedance across its open end, i. e. across the opening of the channel.

Energization of the doublet ll of the antenna 5 by the transmitter l induces currents in the reflector, particularly near the rim. Owing to the high impedance of the trap, substantially none of this induced current flows in the outer portion of the channel l3.

Similarly, any currents flowing in the outer portion of the channel l3 of the receiving antenna l are prevented from inducing currents in the reflector and doublet of the receiving antenna. It has been found by experiment with a typical antenna system that each of the traps decreases the coupling between the transmitter and receiver antennas by about 10 db. Since the attenuations provided by the two traps are su- In the event that more attenuation is required? or operation over a broader frequency band is necessary, one or more trap structures may be added to each antenna, as shown in Figure lfi jwhere additional annular channel members i3 and. I3" are placed outside the channel l3. An-

nular bodies H and M of lossymateriahsucli -i v as rubber, may be provided between the traps,

to provide attenuation which is less fieqtieiiy sensitive than that provided by the channel mem- 9 pets;- The traps is, is" afia [3" 6f aiflfeiii depths, S6 25 th p ovide atthiiatidii over a rati harid Widtli'tlia'rr a sin'gle tram I-cl'ai'iir army invention:

are piererabiy A directive antenna comprising a reflector, a radiator element adjacent said reflector in cooperative relationship therewith, and a plurality of traps surrounding said reflector and adjacent the edge thereof, and separated from each other by bodies of material exhibiting electrical losses of substantially the same order as those characterishe or rubiierj ineach of said traps" comprisin a conductive" bod'y provided with air annular cavity having a depth of the order of one-quarter wavelength at the mean frequency of operation of the system.

v RENE A. BRADEN.

JltliFERENCES CITED The following references are of record in the UNITED STATES PATENTS Number Name Date v 2*;1'022351 Gerliard 15cc. 28; 1937 232815196 llindenblad Apr: 28'; 1942 

